U.S. patent number 8,859,149 [Application Number 13/111,395] was granted by the patent office on 2014-10-14 for anode for lithium ion secondary battery, lithium ion secondary battery, electric power tool, electrical vehicle, and electric power storage system.
This patent grant is currently assigned to Sony Corporation. The grantee listed for this patent is Toshikazu Nakamura. Invention is credited to Toshikazu Nakamura.
United States Patent |
8,859,149 |
Nakamura |
October 14, 2014 |
Anode for lithium ion secondary battery, lithium ion secondary
battery, electric power tool, electrical vehicle, and electric
power storage system
Abstract
A lithium secondary battery that has high capacity and excellent
cycle characteristics is provided. The lithium ion secondary
battery includes a cathode, an anode, and an electrolyte. The anode
has, on an anode current collector, an anode active material layer
including Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and
5.ltoreq.y.ltoreq.6) as an anode active material.
Inventors: |
Nakamura; Toshikazu (Fukushima,
JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
Nakamura; Toshikazu |
Fukushima |
N/A |
JP |
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Assignee: |
Sony Corporation (Tokyo,
JP)
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Family
ID: |
45052970 |
Appl.
No.: |
13/111,395 |
Filed: |
May 19, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110300444 A1 |
Dec 8, 2011 |
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Foreign Application Priority Data
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Jun 3, 2010 [JP] |
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2010-127897 |
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Current U.S.
Class: |
429/231.95;
429/231.4; 429/217; 429/233; 429/232; 252/182.1; 429/231.8;
429/218.1 |
Current CPC
Class: |
H01M
4/364 (20130101); H01M 4/582 (20130101); H01M
4/623 (20130101); H01M 4/136 (20130101); H01M
10/0525 (20130101); H01M 4/38 (20130101); Y02E
60/122 (20130101); H01M 4/485 (20130101); Y02T
10/70 (20130101); H01M 4/386 (20130101); H01M
10/0566 (20130101); H01M 4/608 (20130101); Y02E
60/10 (20130101) |
Current International
Class: |
H01M
4/13 (20100101) |
Field of
Search: |
;429/231.95,218.1,217,231.4,231.8,232,233 ;252/182.1 ;310/50
;320/137 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101872861 |
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Oct 2010 |
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CN |
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1999-250896 |
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Sep 1999 |
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JP |
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2003-0095 |
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Jan 2003 |
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JP |
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2003-217574 |
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Jul 2003 |
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JP |
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2004-228059 |
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Aug 2004 |
|
JP |
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2005-063767 |
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Mar 2005 |
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JP |
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2007-141666 |
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Jun 2007 |
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JP |
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2007-257866 |
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Oct 2007 |
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JP |
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2007-257868 |
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Oct 2007 |
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JP |
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2008-016195 |
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Jan 2008 |
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JP |
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2008-016196 |
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Jan 2008 |
|
JP |
|
Primary Examiner: Weiner; Laura
Attorney, Agent or Firm: K&L Gates LLP
Claims
The application is claimed as follows:
1. An anode for a lithium ion secondary battery comprising: an
anode current collector; and an anode active material layer,
wherein the anode active material layer comprises:
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) and
polyvinylidene fluoride (PVDF).
2. The anode for a lithium ion secondary battery according to claim
1, wherein the Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and
5.ltoreq.y.ltoreq.6) is Li.sub.2SiF.sub.6.
3. The anode for a lithium ion secondary battery according to claim
1, wherein the anode active material layer further comprises
graphite.
4. The anode for a lithium ion secondary battery according to claim
1, wherein the content of polyvinylidene fluoride is 0.1 parts by
weight to 10 parts by weight, both inclusive, when the content of
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) is
100 parts by weight.
5. A lithium ion secondary battery comprising: a cathode; an anode;
and an electrolyte, wherein the anode comprises: an anode current
collector; and an anode active material layer on the anode current
collector, wherein the anode active material layer comprises
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) and
polyvinylidene fluoride (PVDF).
6. The lithium ion secondary battery according to claim 5, wherein
the Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6)
is Li.sub.2SiF.sub.6.
7. The lithium ion secondary battery according to claim 5, wherein
the content of polyvinylidene fluoride is 0.1 parts by weight to 10
parts by weight, both inclusive, when the content of
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) is
100 parts by weight.
8. An electric power tool acting with the use of a lithium ion
secondary battery, wherein the lithium ion secondary battery
comprises: a cathode; an anode; and an electrolytic solution as a
power source; wherein the anode comprises: an anode current
collector; and an anode active material layer on the anode current
collector, wherein the anode active material layer comprises:
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) and
polyvinylidene fluoride (PVDF).
9. An electrical vehicle acting with the use of a lithium ion
secondary battery, wherein the lithium ion secondary battery
comprises: a cathode; an anode; and an electrolytic solution as a
power source, wherein the anode comprises: an anode current
collector; and an anode active material layer on the anode current
collector, wherein the anode active material layer comprises:
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) and
polyvinylidene fluoride (PVDF).
10. An electric power storage system acting with the use of a
lithium ion secondary battery, wherein the lithium ion secondary
battery comprises: a cathode; an anode; and an electrolytic
solution as a power storage source, wherein the anode comprises: an
anode current collector; an anode active material layer on the
anode current collector, wherein the anode active material layer
comprises: Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and
5.ltoreq.y.ltoreq.6) and polyvinylidene fluoride (PVDF).
Description
CROSS REFERENCES TO RELATED APPLICATIONS
The present disclosure claims priority to Japanese Priority Patent
Application JP2010-127897 filed in the Japan Patent Office on Jun.
3, 2010, the entire contents of which is hereby incorporated by
reference.
BACKGROUND
The present disclosure relates to an anode for a lithium ion
secondary battery that contains an anode active material containing
silicon (Si) as an element, a lithium ion secondary battery
including the same, an electric power tool using the lithium ion
secondary battery, an electrical vehicle using the lithium ion
secondary battery, and an electric power storage system using the
lithium ion secondary battery.
In recent years, portable electronic devices such as video cameras,
digital still cameras, mobile phones, and notebook personal
computers have been widely used, and it is strongly demanded to
reduce their size and weight and to achieve their long life.
Accordingly, as a power source for the portable electronic devices,
a battery, in particular a small and light-weight secondary battery
capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of
lithium for charge and discharge reaction (lithium ion secondary
battery) is extremely prospective, since such a secondary battery
is able to provide a higher energy density compared to a lead
battery and a nickel cadmium battery.
The lithium ion secondary battery includes a cathode, an anode, and
an electrolytic solution. The anode has an anode active material
layer provided on an anode current collector. The anode active
material layer contains an anode active material that is involved
in the charge and discharge reaction.
A the anode active material, a carbon material has been widely
used. However, in recent years, as further improvement in battery
capacity is demanded, the use of silicon is being considered. Since
the theoretical capacity of silicon (4199 mAh/g) is significantly
higher than the theoretical capacity of graphite (372 mAh/g), it is
expected that the battery capacity is thereby highly improved. In
this case, the anode active material is not limited to a simple
substance of silicon, and alloys and compounds thereof are also
being considered.
However, in the case where silicon is used as the anode active
material, while the battery capacity improves, some disadvantages
occur. Specifically, since the anode active material intensely
expands and shrinks during charge and discharge, the anode active
material layer is easily pulverized. Further, since reactivity of
the anode active material is high, decomposition of the
electrolytic solution easily occurs.
Therefore, various studies are being conducted to improve various
performances of the lithium ion secondary battery using silicon as
the anode active material.
To improve charge and discharge characteristics, metal particles
(particle size=0.0005 .mu.m to 10 .mu.m, both inclusive) are
provided on a surface of an electrode active material in the
cathode or the anode (for example, see Japanese Unexamined Patent
Application Publication No. Hei 11-250896). To suppress increase in
resistance within the battery and decrease in capacity, a second
active material layer that contains metal and the like that forms
an alloy with the lithium ion, and metal and the like that does not
form an alloy with the lithium ion, is provided on a first active
material layer that inserts and extracts lithium ions (for example,
see Japanese Unexamined Patent Application Publication No.
2003-217574). To improve charge and discharge cycle
characteristics, metal is contained on the surface of a thin film
of which the main constituent is silicon (for example, see Japanese
Unexamined Patent Application Publication No. 2003-007295). To
improve cycle life, a surface coating layer composed of a
conductive material having low lithium-compound forming ability is
provided on an active material layer composed of a silicon material
(for example, see Japanese Unexamined Patent Application
Publication No. 2004-228059). To improve charge and discharge cycle
characteristics, the surfaces of silicon-containing particles
(average particle size (D.sub.50)=0.1 .mu.m to 10 .mu.m, both
inclusive) are coated with a metal thin film (for example, see
Japanese Unexamined Patent Application Publication No.
2005-063767). To obtain excellent charge and discharge efficiency,
an anode material is used in which the surface of a reaction
portion containing silicon is provided with a coated portion
composed of a metal oxide (for example, see Japanese Unexamined
Patent Application Publication No. 2007-141666). To improve
electron conductivity, the anode active material layer contains a
ferromagnetic metal (for example, see Japanese Unexamined Patent
Application Publication No. 2007-257866). In this case, the anode
active material layer has magnetization, and a maximum
magnetization intensity obtained by a magnetization curve is 0.0006
T (tesla) or more. To reduce stress concentration and improve
characteristics, a metal element is contained within the anode
active material layer such that concentration increases and then
decreases in the thickness direction (for example, see Japanese
Unexamined Patent Application Publication No. 2007-257868). To
reduce excess voltage at initial charging, at least a portion of
the surfaces of the particles of the active material is coated with
a metal material having low lithium-compound forming ability (for
example, see Japanese Unexamined Patent Application Publication
Nos. 2008-016195, 2008-016196, 2008-016198, 2008-066278, and
2008-277156).
SUMMARY
The recent portable electronic devices are becoming increasingly
high-performance and multi-functional, and thereby power
consumption thereof tends to increase. In addition, it has been
considered to also apply the lithium ion secondary battery to
large-scale applications, such as electrical vehicles. Accordingly,
it is expected that charge and discharge of the lithium ion
secondary battery will be frequently repeated, and thus the cycle
characteristics will become easily lowered.
view of the foregoing, in the disclosure, it is desirable to
provide an anode for a lithium ion secondary battery capable of
improving the cycle characteristics while being high capacity, a
lithium ion secondary battery using the same, an electric power
tool using the foregoing lithium ion secondary battery, an
electrical vehicle using the foregoing lithium ion secondary
battery, and an electric power storage system using the foregoing
lithium ion secondary battery.
According to an embodiment of the disclosure, there is provided an
anode for a lithium ion secondary battery that has an anode active
material layer including Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and
5.ltoreq.y.ltoreq.6) as an anode active material provided on an
anode current collector. Further, according to an embodiment of the
disclosure, there is provided a lithium ion secondary battery
including the foregoing anode of the disclosure, a cathode, and an
electrolyte. Further, according to an embodiment of the disclosure,
there is provided an electric power tool, an electrical vehicle,
and an electric power storage system that use the foregoing lithium
ion secondary battery as a power source or an electric power
storage source.
According to the anode for a lithium ion secondary battery, and the
lithium ion secondary battery of the embodiments of the disclosure,
the anode active material layer is structured including
Li.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2 and 5.ltoreq.y.ltoreq.6) as
an anode active material. Thereby, volume expansion is able to be
inhibited while increasing the amount of lithium insertion. Thus,
by using silicon as the anode active material, structural breakage
of the anode active material layer accompanying repeated charge and
discharge is able to be inhibited and cycle characteristics are
able to be improved, while actualizing higher capacity. In
addition, according to the electric power tool, the electrical
vehicle, and the electric power storage system of the embodiments
of the disclosure, since a lithium ion secondary battery with
superior cycle characteristics is able to be used, the electric
power tool, the electrical vehicle, and the electric power storage
system are able to be used over a longer period of time.
Additional features and advantages are described herein, and will
be apparent from the following Detailed Description and the
figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a cross sectional view illustrating a structure of an
anode as a first embodiment of the disclosure.
FIG. 2 is a cross sectional view illustrating a structure of a
first secondary battery using the anode of the disclosure.
FIG. 3 is a cross sectional view illustrating an enlarged part of
the spirally wound electrode body illustrated in FIG. 2.
FIG. 4 is an exploded perspective view illustrating a structure of
a second secondary battery using the anode of the disclosure.
FIG. 5 is a cross sectional view illustrating a structure taken
along line V-V of the spirally wound electrode body illustrated in
FIG. 4.
FIG. 6 is a cross sectional view illustrating an enlarged part of
the spirally wound electrode body illustrated in FIG. 5.
FIG. 7 is a cross sectional view illustrating a structure of a
third secondary battery using the anode for a lithium ion secondary
battery of the disclosure.
FIG. 8 is a cross sectional view illustrating a structure taken
along line VIII-VIII of the spirally wound electrode body
illustrated in FIG. 7.
FIG. 9 is a cross sectional view illustrating a structure of a test
cell used in an example of the disclosure.
FIG. 10 is a characteristics diagram illustrating a relationship
between discharge capacity per unit weight and electrode potential
in experiment examples.
FIG. 11 is a characteristics diagram illustrating a relationship
between discharge capacity and number of cycles in the experiment
examples.
DETAILED DESCRIPTION
Embodiments of the present application will be described below in
detail with reference to the drawings.
The description will be given in the following order.
1. First embodiment (anode)
2. Second embodiment (example of a first secondary battery to a
third secondary battery including the foregoing anode)
2-1. First secondary battery (cylindrical type)
2-2. Second secondary battery (laminated film type)
2-3. Third secondary battery (square type)
3. Application of a lithium ion secondary battery
1. First Embodiment
Anode
Whole Structure of an Anode
FIG. 1 illustrates a cross sectional structure of an anode 10 as a
first embodiment of the disclosure. The anode 10 is used in
electrochemical devices, such as lithium ion secondary batteries,
and has an anode current collector 1 having a pair of opposing
faces, and an anode active material layer 2 provided on the anode
current collector 1. The anode active material layer 2 may be
provided on both faces or on one face of the anode current
collector 1.
The anode current collector 1 is preferably made of a metal
material having favorable electrochemical stability, favorable
electric conductivity, and favorable mechanical strength. Examples
of the metal material include copper (Cu), nickel (Ni), and
stainless steel. Specially, copper is preferable as the metal
material, since a high electric conductivity is able to be thereby
obtained.
In particular, the metal material composing the anode current
collector 1 preferably contains one or more metal elements not
forming an intermetallic oxide with an electrode reactant. If the
intermetallic oxide is formed with the electrode reactant, lowering
of the current collectivity characteristics and separation of the
anode active material layer 2 from the anode current collector 1
easily occur, since the anode current collector 1 is broken by
being affected by a stress due to expansion and shrinkage of the
anode active material layer 2 at the time of charge and discharge.
Examples of the metal elements include copper, nickel, titanium
(Ti), iron (Fe), and chromium (Cr).
Further, the foregoing metal material preferably contains one or
more metal elements being alloyed with the anode active material
layer 2. Thereby, the contact characteristics between the anode
current collector 1 and the anode active material layer 2 are
improved, and thus the anode active material layer 2 is less likely
to be separated from the anode current collector 1. For example, in
the case that the anode active material of the anode active
material layer 2 contains silicon (Si), examples of metal elements
that do not form an intermetallic oxide with the electrode reactant
and are alloyed with the anode active material layer 2 include
copper, nickel, and iron. These metal elements are preferable in
terms of the strength and the electric conductivity as well.
The anode current collector 1 may have a single layer structure or
a multilayer structure. In the case where the anode current
collector 1 has the multilayer structure, for example, it is
preferable that the layer adjacent to the anode active material
layer 2 is made of a metal material being alloyed with the anode
active material layer 2, and layers not adjacent to the anode
active material layer 2 are made of other metal material.
The surface of the anode current collector 1 is preferably
roughened. Thereby, due to the so-called anchor effect, the contact
characteristics between the anode current collector 1 and the anode
active material layer 2 are improved. In this case, it is enough
that at least the surface of the anode current collector 1 opposed
to the anode active material layer 2 is roughened. Examples of
roughening methods include a method of forming fine particles by
electrolytic treatment. The electrolytic treatment is a method of
providing concavity and convexity by forming fine particles on the
surface of the anode current collector 1 by electrolytic method in
an electrolytic bath. A copper foil provided with the electrolytic
treatment is generally called "electrolytic copper foil."
Ten point height of roughness profile Rz of the surface of the
anode current collector 1 is, for example, preferably from 1.5
.mu.m to 6.5 .mu.m both inclusive, since thereby the contact
characteristics between the anode current collector 1 and the anode
active material layer 2 are more improved.
Anode Active Material Layer
The anode active material layer 2 is structured to contain, as an
anode active material, one or more types of an anode material
capable of inserting and extracting lithium that is an electrode
reactant. The anode active material layer 2 may contain other
material, such as an anode electrical conductor or an anode binder,
in addition to the foregoing anode active material according to
needs. Details regarding the anode electrical conductor and the
anode binder are similar, for example, to those of a cathode
conductive agent and a cathode binder, described hereafter.
The anode active material layer 2 contains a compound expressed by
M.sub.xSiF.sub.y (1.ltoreq.x.ltoreq.2, 5.ltoreq.y.ltoreq.6) as such
an anode material. Preferably, the anode active material layer 2
contains lithium fluorosilicate (Li.sub.xSiF.sub.y
(1.ltoreq.x.ltoreq.2, 5.ltoreq.y.ltoreq.6)).
For example, Li.sub.2SiF.sub.6 has an octahedron structure in which
fluorine atoms are arranged around a silicon element positioned in
the center. Since the lithium ions are coordinated near the
fluorine atoms, electron density of the fluorine atoms is affected
by movement of electrons to the silicon atom. Thereby, insertion
and extraction of lithium is smoothly repeated. Further,
Li.sub.2SiF.sub.6 is a solid powder, and solubility to the
non-aqueous electrolyte solvent used in the lithium ion secondary
battery is extremely low. Even if the fluorine atoms become five as
a result of the state of valence modification of silicon,
Li.sub.2SiF.sub.6 does not liquefy, and is therefore suitable as
the anode active material in terms of stability as well.
In addition, a material (metal material) in which at least one type
of metal element or metalloid element is an element may also be
contained as the anode material, since thereby a high energy
density is able to be obtained. The metal material may be a simple
substance, an alloy, or a compound of the metal element or
metalloid element; or a metal material having one or more phases
thereof at least in part.
The foregoing metal element or metalloid element is, for example, a
metal element or a metalloid element capable of forming an alloy
with the electrode reactant, and is specifically one or more of the
following elements. That is, the foregoing metal element or the
foregoing metalloid element is one or more of magnesium (Mg), boron
(B), aluminum (Al), gallium (G), indium (In), germanium (Ge), tin
(Sn), lead (Pb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc
(Zn), hafnium (Hf), zirconium (Zr), yttrium (Y), palladium (Pd),
and platinum (Pt). Specially, silicon or tin is preferable, and
silicon is more preferable, since further high energy density is
able to be obtained.
A material containing silicon (silicon-containing material) may be
a simple substance, an alloy, or a compound of silicon; or a
material having one or more phases thereof at least in part.
Examples of alloys of silicon include a material having one or more
of the following elements as an element other than silicon. Such an
element other than silicon is tin, nickel, copper, iron (Fe),
cobalt (Co), manganese (Mn), zinc, indium, silver, titanium (Ti),
germanium, bismuth, antimony (Sb), and chromium (Cr). Examples of
compounds of silicon include a compound containing oxygen (O) or
carbon (C) as an element other than silicon. The compounds of
silicon may have one or more of the elements described for the
alloys of silicon as an element other than silicon. Examples of an
alloy or a compound of silicon include SiB.sub.4, SiB.sub.6,
Mg.sub.2Si, Ni.sub.2Si, TiSi.sub.2, MoSi.sub.2, CoSi.sub.2,
NiSi.sub.2, CaSi.sub.2, CrSi.sub.2, Cu.sub.5Si, FeSi.sub.2,
MnSi.sub.2, NbSi.sub.2, TaSi.sub.2, VSi.sub.2, WSi.sub.2,
ZnSi.sub.2, SiC, Si.sub.3N.sub.4, Si.sub.2N.sub.2O, SiO.sub.v
(0<v.ltoreq.2), SnO.sub.w (0<w.ltoreq.2), and LiSiO.
A material containing tin (tin-containing material) may be a simple
substance, an alloy, or a compound of tin; or a material having one
or more phases thereof at least in part. Examples of alloys of tin
include a material having one or more of the following elements as
an element other than tin. Such an element other than tin is
silicon, nickel, copper, iron, cobalt, manganese, zinc, indium,
silver, titanium, germanium, bismuth, antimony, and chromium.
Examples of compounds of tin include a compound containing oxygen
(O) or carbon (C) as an element other than tin. The compounds of
tin may have one or more of the elements described for the alloys
of tin as an element other than tin. Examples of an alloy or a
compound of tin include SnSiO.sub.3, LiSnO, and Mg.sub.2Sn.
In particular, for example, the tin-containing material preferably
contains a second element and a third element in addition to tin as
a first element, since thereby a high energy density is able to be
stably obtained. The second element is, for example, one or more of
the following elements. That is, the second element is one or more
of cobalt, iron, magnesium, titanium, vanadium, chromium,
manganese, nickel, copper, zinc, gallium, zirconium, niobium,
molybdenum, silver, indium, cerium (Ce), hafnium, tantalum,
tungsten, bismuth, and silicon. The third element is, for example,
one or more of boron, carbon, aluminum, and phosphorus.
Other anode material is, for example, a carbon material, since
crystal structure change at the time of insertion and extraction of
the electrode reactant is extremely small, and high energy density
is able to be obtained. In addition, the carbon material functions
as an anode electrical conductor. Examples of carbon materials
include graphitizable carbon, non-graphitizable carbon in which the
spacing of (002) plane is 0.37 nm or more, and graphite in which
the spacing of (002) plane is 0.34 nm or less. More specifically,
examples of carbon materials include pyrolytic carbon, coke, glassy
carbon fiber, an organic polymer compound fired body, activated
carbon, and carbon black. The coke includes pitch coke, needle
coke, and petroleum coke. The organic polymer compound fired body
is obtained by firing and carbonizing a phenol resin, a furan resin
or the like at appropriate temperature. The shape of the carbon
material may be any of a fibrous shape, a spherical shape, a
granular shape, and a scale-like shape.
In addition, examples of other anode materials include a metal
oxide and a polymer compound. Examples of the metal oxide include
iron oxide, ruthenium oxide, and molybdenum oxide. Examples of the
polymer compound include polyacetylene, polyaniline, and
polypyrrole.
It is needless to say that the anode material may be a material
other than the foregoing. Further, two or more of the foregoing
anode active materials may be used by mixture arbitrarily.
The anode active material layer 2 is formed by, for example,
coating method, vapor-phase deposition method, liquid-phase
deposition method, spraying method, firing method (sintering
method), or a combination of two or more of these methods. Coating
method is a method in which, for example, a particulate anode
active material is mixed with an anode binder or the like, the
mixture is dispersed in a solvent, and the anode current collector
is coated with the resultant. Examples of vapor-phase deposition
methods include physical deposition method and chemical deposition
method. Specifically, examples thereof include vacuum evaporation
method, sputtering method, ion plating method, laser ablation
method, thermal CVD (Chemical Vapor Deposition) method, and plasma
CVD method. Examples of liquid-phase deposition methods include
electrolytic plating method and electroless plating method.
Spraying method is a method in which the anode active material is
sprayed in a fused state or a semi-fused state. Firing method is,
for example, a method in which after the anode current collector is
coated by a procedure similar to that of coating method, heat
treatment is provided at a temperature higher than the melting
point of the anode binder or the like. Examples of firing methods
include a known technique such as atmosphere firing method,
reactive firing method, and hot press firing method. Specially, the
anode active material layer 2 is preferably formed by the
vapor-phase deposition method since the anode active material layer
2 is bonded to the anode current collector 1, thereby increasing
denseness of the anode active material layer 2.
Further, a method for forming the anode active material layer 2
includes, in addition to the foregoing methods, mechanical alloying
method, a method in which raw material compounds are mixed and
heat-treated in inert atmosphere, melt spinning method, gas
atomization method, and water atomization method. Still further, in
the case where lithium fluorosilicate is used as the anode
material, it is possible to react hydrofluorosilicic acid
(hexafluorosilicic acid: H.sub.2SiF.sub.6) with an alkaline
solution containing lithium hydroxide (LiOH) or the like, and
salt-out lithium fluorosilicate. Each material may be pulverized or
unpulverized, and two or more materials may be combined.
Manufacturing Method of the Anode
The anode 10 is manufactured, for example, by the following
procedure.
Specifically, first, the anode current collector 1 is prepared, and
the surface of the anode current collector 1 is provided with
roughening treatment according to needs. After that, the anode
active material containing the foregoing anode material is
deposited on the surface of the anode current collector 1 using the
foregoing methods, such as vapor-phase deposition method, thereby
the anode active material layer 2 is formed. If vapor-phase
deposition method is used, the anode active material may be
deposited while the anode current collector 1 is fixed, or the
anode active material may be deposited while the anode current
collector 1 is rotated. Thereby, the anode 10 is completed.
Action and Effect of this Embodiment
As described above, according to this embodiment, the anode active
material layer 2 contains lithium fluorosilicate as the anode
active material. Therefore, in the case where the anode 10 is used
in a lithium ion secondary battery or the like, stress resulting
from expansion and shrinkage at the time of charge and discharge in
the anode active material layer 2 is relaxed. As a reason for
expansion accompanying charge and discharge in the anode active
material layer, increase in space between silicon atoms
accompanying lithium inserted and excessive formation of a
solid-liquid interface layer (SEI) coating in the periphery of the
silicon atoms are considered. Since a more significant expansion
occurs after charge and discharge cycles are repeatedly performed,
compared to initial charge and discharge, it is presumed that the
expansion attributed to the latter (expansion due to excessive
formation of SEI coating) has a greater effect than expansion
attributed to the former. In the lithium fluorosilicate used as the
anode active material in this embodiment, since bonds surrounding
silicon are all attached to fluorine or lithium, a new bond with a
material that may become SEI coating, such as an organic matter or
a carbonic acid root having a long chain length, is not formed.
Therefore, since the SEI coating is not excessively formed,
expansion of the anode active material layer 2 caused by repeated
charge and discharge is inhibited, and stable insertion and
extraction of lithium ions is continued. Therefore, structural
breakage of the anode active material layer 2 is inhibited, and
contact characteristics between the anode active material layer 2
and the anode current collector 1, and current collectivity are
improved. As a result, in the case where the anode 10 is applied to
the lithium ion secondary battery, superior cycle characteristics
are able to be obtained while realizing high capacity. In addition,
in the case where graphite is also included with Li.sub.2SiF.sub.6
as the anode active material, since increase in resistance
accompanying the addition of an insulating binder is able to be
suppressed, higher capacity and improvement of cycle
characteristics are expected. Further, compared to a case where
only Li.sub.2SiF.sub.6 is used as the anode active material,
decrease in electric potential of the anode 10 at the time of
charge is able to be relaxed, and thereby micro-short circuits
become very rare occurrences.
Modification
Next, a modification of the anode 10 according to the foregoing
first embodiment will be described.
The anode active material layer may contain a fluorine-containing
compound having a negative charge or an unshared electron pair, and
having a constant degree of polymerization or structural sterical
hinder. Specifically, examples of such fluorine-containing compound
include polyvinylidene fluoride (PVDF),
tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA),
tetrafluoroethylene-hexafluoropropylene copolymer (PFA), and
polytetrafluoroethylene (PTFE). In this case, the anode 10 is
manufactured, for example, in the following manner. Specifically,
an anode active material containing lithium fluorosilicate and the
like and the foregoing fluorine-containing compound are mixed in a
solution state and an anode mixture is prepared. The anode mixture
is then dispersed in a solvent, such as N-methyl-2-pyrrolidone
(NMP), and paste anode mixture slurry is obtained. Next, the anode
current collector 1 is coated with the anode mixture slurry, and
the solvent is dried. Then, the anode active material layer 2 is
formed by compression molding by a rolling press machine or the
like, thereby obtaining the anode 10.
In the modification, in the case where the anode active material
layer 2 contains lithium fluorosilicate as the anode material and
polyvinylidene fluoride as the fluorine-containing compound, the
content of polyvinylidene fluoride is preferably 0.1 parts by
weight to 10 parts by weight, both inclusive, when the content of
lithium fluorsilicate is 100 parts by weight.
In addition, to manufacture the anode active material layer 2, the
anode mixture slurry may be formed by the anode mixture in which
the foregoing anode active material and the foregoing
fluorine-containing compound are mixed being dispersed in a
solution containing one or more of styrene-butadiene rubber,
acrylic rubber, methyl methacrylate, carboxymethylcellulose,
methylcellulose, and hydroxyopropyl methylcellulose instead of
N-methyl-2-pyrrolidone.
In this way, in the modification, the anode active material layer 2
contains a given fluorine-containing compound. Therefore, in the
case where the anode 10 is used in a lithium ion secondary battery
or the like, decomposition reaction of the anode active material
caused by acidic impurities, such as hydrogen fluoride (HF), within
the electrolytic solution is able to be inhibited. Ordinarily, when
the foregoing acidic impurity present in the electrolytic solution
acts, for example, on the lithium atoms in Li.sub.2SiF.sub.6 having
the octahedron structure, SiF.sub.4 and HF are newly generated by
the decomposition reaction thereof. Decomposition reaction such as
this may cause various issues, such as decrease in discharge
capacity of the battery, increase in internal resistance, and
decrease in cycle life. Therefore, in the modification, as a result
of the effect of the fluorine-containing compound included in the
anode active material layer 2, reaction between the lithium atoms
in lithium fluorosilicate and the acidic impurity is inhibited.
Specifically, as a result of a state in which the lithium
fluorosilicate and the fluorine-containing compound are dispersed
in solvent such as N-methyl-2-pyrrolidone being obtained (anode
mixture slurry being formed), .delta.+ of the lithium atom in
lithium fluorosilicate and .delta.- of the fluorine atom in the
fluorine-containing compound are attracted, and reaction between
the lithium atoms and the acidic impurity is interfered.
As described above, in the modification, further higher capacity
and further improvement in cycle characteristics are able to be
obtained by sufficiently inhibiting the decomposition reaction of
the anode active material.
2. Second Embodiment
Next, a description will be given of usage examples of the anode 10
described in the foregoing first embodiment. A description will be
given taking as an example a first secondary battery to a third
secondary battery as a lithium ion secondary battery for which the
anode 10 is used.
2-1. First Secondary Battery (Cylindrical Type)
FIG. 2 and FIG. 3 illustrate a cross sectional structure of a first
secondary battery. FIG. 3 illustrates an enlarged part of a
spirally wound electrode body 20 illustrated in FIG. 2. The
secondary battery herein described is, for example, a lithium ion
secondary battery in which, for example, a capacity of an anode 22
is expressed based on insertion and extraction of lithium.
Whole Structure of the First Secondary Battery
The secondary battery mainly contains the spirally wound electrode
body 20 in which a cathode 21 and the anode 22 are layered with a
separator 23 in between and spirally wound, and a pair of
insulating plates 12 and 13 inside a battery can 11 in the shape of
an approximately hollow cylinder. The battery structure including
the battery can 11 is a so-called cylindrical type.
The battery can 11 is made of, for example, a metal material such
as iron, aluminum, or an alloy thereof. One end of the battery can
11 is closed, and the other end of the battery can 11 is opened.
The pair of insulating plates 12 and 13 is arranged to sandwich the
spirally wound electrode body 20 in between and to extend
perpendicularly to the spirally wound periphery face.
At the open end of the battery can 11, a battery cover 14, and a
safety valve mechanism 15 and a PTC (Positive Temperature
Coefficient) device 16 provided inside the battery cover 14 are
attached by being caulked with a gasket 17. Inside of the battery
can 11 is thereby hermetically sealed. The battery cover 14 is made
of, for example, a material similar to that of the battery can 11.
The safety valve mechanism 15 is electrically connected to the
battery cover 14 through the PTC device 16. In the safety valve
mechanism 15, in the case where the internal pressure becomes a
certain level or more by internal short circuit, external heating
or the like, a disk plate 15A flips to cut the electric connection
between the battery cover 14 and the spirally wound electrode body
20. As temperature rises, the PTC device 16 increases the
resistance and thereby limits a current to prevent abnormal heat
generation resulting from a large current. The gasket 17 is made
of, for example, an insulating material. The surface of the gasket
17 is coated with asphalt.
A center pin 24 may be inserted in the center of the spirally wound
electrode body 20. In the spirally wound electrode body 20, a
cathode lead 25 made of a metal material such as aluminum is
connected to the cathode 21, and an anode lead 26 made of a metal
material such as nickel is connected to the anode 22. The cathode
lead 25 is electrically connected to the battery cover 14 by being
welded to the safety valve mechanism 15. The anode lead 26 is
welded and thereby electrically connected to the battery can
11.
Cathode
The cathode 21 has a structure in which, for example, a cathode
active material layer 21B is provided on both faces of a cathode
current collector 21A having a pair of faces. The cathode current
collector 21A is made of a metal material such as aluminum, nickel,
and stainless steel. The cathode active material layer 21B contains
a cathode active material, and may contain other material such as a
binder and an electrical conductor according to needs.
The cathode active material contains one or more cathode materials
capable of inserting and extracting lithium as an electrode
reactant. As the cathode material, for example, a
lithium-containing compound is preferable, since thereby a high
energy density is able to be obtained. Examples of the
lithium-containing compound include a composite oxide containing
lithium and a transition metal element, and a phosphate compound
containing lithium and a transition metal element. Specially, a
compound containing at least one selected from the group consisting
of cobalt, nickel, manganese, and iron as a transition metal
element is preferable, since thereby a higher voltage is able to be
obtained. The chemical formula thereof is expressed by, for
example, Li.sub.xM1O.sub.2 or Li.sub.yM2PO.sub.4. In the formula,
M1 and M2 represent one or more transition metal elements. Values
of x and y vary according to the charge and discharge state of the
secondary battery, and are generally in the range of
0.05.ltoreq.x.ltoreq.1.10 and 0.05.ltoreq.y.ltoreq.1.10.
Examples of composite oxides containing lithium and a transition
metal element include a lithium cobalt composite oxide
(Li.sub.xCoO.sub.2), a lithium nickel composite oxide
(Li.sub.xNiO.sub.2), a lithium nickel cobalt composite oxide
(Li.sub.xNi.sub.(1-z)CO.sub.zO.sub.2 (z<1)), a lithium nickel
cobalt manganese composite oxide
(Li.sub.xNi.sub.(1-v-w)CO.sub.vMn.sub.wO.sub.2) (v+w<1)), and a
lithium manganese composite oxide having a spinel structure
(LiMn.sub.2O.sub.4). Specially, a composite oxide containing cobalt
is preferable, since thereby a high capacity is obtained and
superior cycle characteristics are obtained. Further, examples of
phosphate compounds containing lithium and a transition metal
element include lithium iron phosphate compound (LiFePO.sub.4) and
a lithium iron manganese phosphate compound
(LiFe.sub.(1-u)Mn.sub.uPO.sub.4(u<1)).
In addition, examples of cathode materials include an oxide, a
disulfide, a chalcogenide, and a conductive polymer. Examples of
oxides include titanium oxide, vanadium oxide, and manganese
dioxide. Examples of disulfides include titanium disulfide and
molybdenum sulfide. Examples of chalcogenide include niobium
selenide. Examples of conductive polymers include sulfur,
polyaniline and polythiophene.
It is needless to say that the cathode material may be a material
other than the foregoing compounds. Further, two or more of the
foregoing cathode materials may be used by mixture arbitrarily.
Examples of cathode binders include a synthetic rubber such as
styrene-butadiene rubber, fluorine system rubber, and ethylene
propylenediene, and a polymer material such as polyvinylidene
fluoride. One thereof may be used singly, or a plurality thereof
may be used by mixture.
Examples of cathode electrical conductors include a carbon material
such as graphite, carbon black, acetylene black, and Ketjen black.
One thereof may be used singly, or a plurality thereof may be used
by mixture. The cathode electrical conductor may be a metal
material, a conductive polymer or the like as long as the material
has electric conductivity.
Anode
The anode 22 has a structure similar to that of the foregoing anode
10. For example, in the anode 22, an anode active material layer
22B is provided on both faces of an anode current collector 22A
having a pair of faces. The structures of the anode current
collector 22A and the anode active material layer 22B are
respectively similar to the structures of the anode current
collector 1 and the anode active material layer 2 in the foregoing
anode. In the anode 22, the chargeable capacity of the anode
material capable of inserting and extracting lithium is preferably
larger than the chargeable capacity of the cathode 21. Thereby, at
the time of full charge, there is low possibility that lithium is
precipitated as dendrite on the anode 22.
Separator
The separator 23 separates the cathode 21 from the anode 22, and
passes lithium ions while preventing current short circuit due to
contact of both electrodes. The separator 23 is made of, for
example, a porous film composed of a synthetic resin such as
polytetrafluoroethylene, polypropylene, and polyethylene, or a
ceramics porous film. The separator 23 may have a structure in
which two or more porous films are layered. Specially, a porous
film made of polyolefin is preferable, since such a film has
superior short circuit preventive effect, and is able to achieve
safety improvement of the secondary battery by shutdown effect. In
particular, polyethylene is preferable since shutdown effect is
able to be thereby obtained at from 100 deg C. to 160 deg C. both
inclusive and its electrochemical stability is excellent. Further,
polypropylene is also preferable. In addition, a copolymer of
polyethylene and polypropylene or a blended material thereof may be
used as long as such a resin has chemical stability.
Electrolytic Solution
An electrolytic solution as a liquid electrolyte impregnates the
separator 23. The electrolytic solution contains a solvent and an
electrolyte salt dissolved therein.
The solvent contains, for example, one or more nonaqueous solvents
such as an organic solvent. The solvents (nonaqueous solvents)
described below may be used singly or two or more thereof may be
used by mixture.
Examples of nonaqueous solvents include ethylene carbonate,
propylene carbonate, butylene carbonate, dimethyl carbonate,
diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate,
.gamma.-butyrolactone, .gamma.-valerolactone, 1,2-dimethoxyethane,
tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran,
1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane,
methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,
methyl butyrate, methyl isobutyrate, trimethylacetic acid methyl,
trimethylacetic acid ethyl, acetonitrile, glutaronitrile,
adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile,
N,N-dimethylformamide, N-methylpyrrolidinone,
N-methyloxazolidinone, N,N'-dimethylimidazolidinone, nitromethane,
nitroethane, sulfolane, trimethyl phosphate, and dimethyl
sulfoxide. By using such a nonaqueous solvent, a superior battery
capacity, superior cycle characteristics, superior storage
characteristics and the like are obtained.
Specially, at least one of ethylene carbonate, propylene carbonate,
dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate
is preferable. By using such a nonaqueous solvent, a superior
battery capacity, superior cycle characteristics, superior storage
characteristics and the like are obtained. In this case, a mixture
of a high viscosity (high dielectric constant) solvent (for
example, specific inductive .di-elect cons..gtoreq.30) such as
ethylene carbonate and propylene carbonate and a low viscosity
solvent (for example, viscosity.ltoreq.1 mPas) such as dimethyl
carbonate, ethylmethyl carbonate, and diethyl carbonate is more
preferable. Thereby, dissociation characteristics of the
electrolyte salt and ion mobility are improved.
In particular, the solvent preferably contains at least one of a
halogenated chain ester carbonate and a halogenated cyclic ester
carbonate. Thereby, a stable protective film is formed on the
surface of the anode 22 at the time of charge and discharge, and
thus decomposition reaction of the electrolytic solution is
inhibited. The halogenated chain ester carbonate is a chain ester
carbonate having halogen as an element. More specifically, at least
some of hydrogen in the chain ester carbonate are substituted with
halogen. Further, the halogenated cyclic ester carbonate is a
cyclic ester carbonate containing halogen as an element. More
specifically, at least some of hydrogen in the cyclic ester
carbonate are substituted with halogen.
The halogen type is not particularly limited, but specially,
fluorine, chlorine, or bromine is preferable, and fluorine is more
preferable since thereby higher effect is obtained compared to
other halogen. The number of halogen is more preferably two than
one, and further may be three or more, since thereby an ability to
form a protective film is improved, and a more rigid and more
stable protective film is formed. Accordingly, decomposition
reaction of the electrolytic solution is more inhibited.
Examples of the halogenated chain ester carbonate include
fluoromethyl methyl carbonate, bis(fluoromethyl) carbonate, and
difluoromethyl methyl carbonate. Examples of the halogenated cyclic
ester carbonate include 4-fluoro-1,3-dioxolane-2-one and
4,5-difluoro-1,3-dioxolane-2-one. Halogenated cyclic ester
carbonate includes a geometric isomer as well. Contents of the
halogenated chain ester carbonate and the halogenated cyclic ester
carbonate in the solvent is, for example, from 0.01 wt % to 50 wt %
both inclusive.
Further, the solvent preferably contains an unsaturated carbon bond
cyclic ester carbonate. Thereby, a stable protective film is formed
on the surface of the anode 22 at the time of charge and discharge,
and thus decomposition reaction of the electrolytic solution is
inhibited. The unsaturated carbon bond cyclic ester carbonate is a
cyclic ester carbonate having an unsaturated carbon bond. More
specifically, unsaturated carbon bond is introduced to a given
location of the cyclic ester carbonate. Examples of the unsaturated
carbon bond cyclic ester carbonate include vinylene carbonate and
vinylethylene carbonate. Contents of the unsaturated carbon bond
cyclic ester carbonate in the solvent is, for example, from 0.01 wt
% to 10 wt % both inclusive.
Further, the solvent preferably contains sultone (cyclic sulfonic
ester), since thereby chemical stability of the electrolytic
solution is improved. Examples of the sultone include propane
sultone and propene sultone. The sultone content in the solvent is,
for example, from 0.5 wt % to 5 wt % both inclusive.
Further, the solvent preferably contains an acid anhydride since
chemical stability of the electrolytic solution is thereby
improved. Examples of acid anhydrides include carboxylic anhydride,
disulfonic anhydride, and carboxylic sulfonic anhydride. Examples
of carboxylic anhydrides include succinic anhydride, glutaric
anhydride, and maleic anhydride. Examples of disulfonic anhydrides
include ethane disulfonic anhydride and propane disulfonic
anhydride. Examples of carboxylic sulfonic anhydrides include
sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric
anhydride. The content of the acid anhydride in the solvent is, for
example, from 0.5 wt % to 5 wt % both inclusive.
The electrolyte salt contains, for example, one or more light metal
salts such as a lithium salt. The electrolyte salts described below
may be used singly or two or more thereof may be used by
mixture.
Examples of lithium salts include the following. That is, examples
thereof include lithium hexafluorophosphate (LiPF.sub.6), lithium
tetrafluoroborate (LiBF.sub.4), lithium perchlorate (LiClO.sub.4),
and lithium hexafluoroarsenate (LiAsF.sub.6). Further, examples
thereof include lithium tetraphenylborate
(LiB(C.sub.6H.sub.5).sub.4), lithium methanesulfonate
(LiCH.sub.3SO.sub.3), lithium trifluoromethane sulfonate
(LiCF.sub.3SO.sub.3), and lithium tetrachloroaluminate
(LiAlCl.sub.4). Further, examples thereof include dilithium
hexafluorosilicate (Li.sub.2SiF.sub.6), lithium chloride (LiCl),
and lithium bromide (LiBr). In the case of using the foregoing
material, superior battery capacity, superior cycle
characteristics, superior storage characteristics and the like are
obtained.
Specially, at least one of lithium hexafluorophosphate, lithium
tetrafluoroborate, lithium perchlorate, and lithium
hexafluoroarsenate is preferable. Further, lithium
hexafluorophosphate and lithium tetrafluoroborate are more
preferable, and lithium hexafluorophosphate is most preferable,
since the internal resistance is thereby lowered, more superior
effect is obtained.
The content of the electrolyte salt to the solvent is preferably
from 0.3 mol/kg to 3.0 mol/kg both inclusive, since thereby high
ion conductivity is obtained.
The electrolytic solution may contain various additives together
with the solvent and the electrolyte salt, since thereby chemical
stability of the electrolytic solution is more improved.
Examples of additives include sultone (cyclic ester sulfonate).
Examples of sultone include propane sultone and propene sultone.
Specially, propene sultone is preferable. Such sultone may be used
singly, or a plurality thereof may be used by mixture.
Examples of additives include an acid anhydride. Examples of acid
anhydrides include carboxylic anhydride such as succinic anhydride,
glutaric anhydride, and maleic anhydride; disulfonic anhydride such
as ethane disulfonic anhydride and propane disulfonic anhydride;
and an anhydride of carboxylic acid and sulfonic acid such as
sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric
anhydride. Specially, sulfobenzoic anhydride or sulfopropionic
anhydride is preferable. The anhydrides may be used singly, or a
plurality thereof may be used by mixture.
Manufacturing Method of the Secondary Battery
The secondary battery is manufactured, for example, by the
following procedure.
First, the cathode 21 is formed. First, a cathode active material,
and if necessary, a cathode binder, a cathode electrical conductor
or the like are mixed to prepare a cathode mixture, which is
dispersed in an organic solvent to form paste cathode mixture
slurry. Subsequently, both faces of the cathode current collector
21A are uniformly coated with the cathode mixture slurry, which is
dried to form the cathode active material layer 21B. Finally, the
cathode active material layer 21B is compression-molded by using a
rolling press machine or the like while being heated if necessary.
In this case, the resultant may be compression-molded over several
times.
Next, the anode 22 is formed by a procedure similar to that of the
foregoing anode 10 or the like. In this case, after the anode
current collector 22A is prepared, the anode active material layer
22B is formed by sequentially forming a first region, an
oxygen-containing region, and a second region on both faces of the
anode current collector 22A.
Finally, the secondary battery is assembled by using the cathode 21
and the anode 22. First, the cathode lead 25 is attached to the
cathode current collector 21 by welding or the like, and the anode
lead 26 is attached to the anode current collector 22A by welding
or the like. Subsequently, the cathode 21 and the anode 22 are
layered with the separator 23 in between and spirally wound, and
thereby the spirally wound electrode body 20 is formed. After that,
the center pin 24 is inserted in the center of the spirally wound
electrode body. Subsequently, the spirally wound electrode body 20
is sandwiched between the pair of insulating plates 12 and 13, and
contained in the battery can 11. In this case, the cathode lead 25
is attached to the safety valve mechanism 15 by welding or the
like, and the anode lead 26 is attached to the battery can 11 by
welding or the like. Subsequently, the electrolytic solution is
injected into the battery can 11 and impregnates the separator 23.
Finally, after the battery cover 14, the safety valve mechanism 15,
and the PTC device 16 are attached to the open end of the battery
can 11, the resultant is caulked with the gasket 17. Thereby, the
secondary battery illustrated in FIG. 2 and FIG. 3 is
completed.
The anode active material composing the anode active material layer
22 may be electrochemically doped with lithium inside the battery
after the battery is formed, may be electrochemically doped by
being supplied lithium from the cathode or a lithium source other
than the cathode before or after the battery is formed, or the
anode active material may be synthesized as a lithium-containing
compound during material synthesis and lithium may be contained in
the anode at the time the battery is formed.
Operation of the Secondary Battery
In the secondary battery, when charged, for example, lithium ions
are extracted from the cathode 21 and inserted in the anode 22
through the electrolytic solution impregnating the separator 23.
Meanwhile, when discharged, for example, lithium ions are extracted
from the anode 22, and inserted in the cathode 21 through the
electrolytic solution impregnating the separator 23.
Effect of the Secondary Battery
According to the first secondary battery, the anode 22 has the
structure similar to that of the anode 10 illustrated in FIG. 1.
Thus, the cycle characteristics are able to be improved while a
high capacity is obtained. Effects of the first secondary battery
other than the foregoing effects are similar to those of the
foregoing anode 10.
2-2. Second Secondary Battery (Laminated Film Type)
FIG. 4 illustrates an exploded perspective structure of a second
secondary battery. FIG. 5 illustrates an enlarged cross section
taken along line V-V of a spirally wound electrode body 30
illustrated in FIG. 4.
The secondary battery is, for example, a lithium ion secondary
battery as the first secondary battery. In the second secondary
battery, a spirally wound electrode body 30 on which a cathode lead
31 and an anode lead 32 are attached is contained in a film package
member 40. The battery structure using the package member 40 is
so-called laminated film type.
The cathode lead 31 and the anode lead 32 are respectively directed
from inside to outside of the package member 40 in the same
direction, for example. However, positions of the cathode lead 31
and the anode lead 32 to the spirally wound electrode body 30, the
derivation direction thereof and the like are not particularly
limited. The cathode lead 31 is made of, for example, aluminum or
the like, and the anode lead 32 is made of, for example, copper,
nickel, stainless steel or the like. These materials are in the
shape of a thin plate or mesh.
The package member 40 is a laminated film in which, for example, a
fusion bonding layer, a metal layer, and a surface protective layer
are layered in this order. In this case, for example, the
respective outer edges in the fusion bonding layer of two films are
bonded to each other by fusion bonding, an adhesive or the like so
that the fusion bonding layer and the spirally wound electrode body
30 are opposed to each other. Examples of fusion bonding layers
include a film made of polyethylene, polypropylene or the like.
Examples of metal layers include an aluminum foil. Examples of
surface protective layers include a film made of nylon,
polyethylene terephthalate or the like.
Specially, as the package member 40, an aluminum laminated film in
which a polyethylene film, an aluminum foil, and a nylon film are
layered in this order is preferable. However, the package member 40
may be made of a laminated film having other laminated structure, a
polymer film such as polypropylene, or a metal film instead of the
foregoing aluminum laminated film.
An adhesive film 41 to protect from entering of outside air is
inserted between the package member 40 and the cathode lead 31, the
anode lead 32. The adhesive film 41 is made of a material having
contact characteristics with respect to the cathode lead 31 and the
anode lead 32. Examples of such a material include, for example, a
polyolefin resin such as polyethylene, polypropylene, modified
polyethylene, and modified polypropylene.
In the spirally wound electrode body 30, as illustrated in FIG. 5 a
cathode 33 and an anode 34 are layered with a separator 35 and an
electrolyte layer 36 in between and spirally wound. The outermost
periphery thereof is protected by a protective tape 37. The cathode
33 has a structure in which, for example, a cathode active material
layer 33B is provided on both faces of a cathode current collector
33A. The anode 34 has a structure in which, for example, an anode
active material layer 34B is provided on both faces of an anode
current collector 34A.
FIG. 6 illustrates an enlarged part of the spirally wound electrode
body 30 illustrated in FIG. 5. The cathode 33 has a structure in
which, for example, the cathode active material layer 33B is
provided on both faces of the cathode current collector 33A having
a pair of faces. The anode 34 has a structure similar to that of
the foregoing anode in which, for example, the anode active
material layer 34B is provided on both faces of the anode current
collector 34A having a pair of faces. The structures of the cathode
current collector 33A, the cathode active material layer 33B, the
anode current collector 34A, the anode active material layer 34B,
and the separator 35 are respectively similar to those of the
cathode current collector 21A, the cathode active material layer
21B, the anode current collector 22A, the anode active material
layer 22B, and the separator 23 in the foregoing first secondary
battery.
In the electrolyte layer 36, an electrolytic solution is held by a
polymer compound. The electrolyte layer 36 may contain other
material such as various additives according to needs. The
electrolyte layer 36 is a so-called gel electrolyte. The gel
electrolyte is preferable, since high ion conductivity (for
example, 1 mS/cm or more at room temperature) is obtained and
liquid leakage of the electrolytic solution is prevented.
Examples of polymer compounds include one or more of the following
polymer materials. That is, examples thereof include
polyacrylonitrile, polyvinylidene fluoride,
polytetrafluoroethylene, polyhexafluoropropylene, polyethylene
oxide, polypropylene oxide, polyphosphazene, polysiloxane, and
polyvinyl fluoride. Further, examples thereof include polyvinyl
acetate, polyvinyl alcohol, polymethacrylic acid methyl,
polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber,
nitrile-butadiene rubber, polystyrene, and polycarbonate. Further,
examples thereof include a copolymer of vinylidene fluoride and
hexafluoropropylene. Such a compound may be used singly, or a
plurality thereof may be used by mixture. Specially, polyvinylidene
fluoride or the copolymer of vinylidene fluoride and
hexafluoropropylene is preferable, since such a polymer compound is
electrochemically stable.
The composition of the electrolytic solution is similar to the
composition of the electrolytic solution in the first secondary
battery. However, in the electrolyte layer 36 as the gel
electrolyte, a solvent of the electrolytic solution means a wide
concept including not only the liquid solvent but also a material
having ion conductivity capable of dissociating the electrolyte
salt. Therefore, in the case where the polymer compound having ion
conductivity is used, the polymer compound is also included in the
solvent.
Instead of the gel electrolyte layer 36 in which an electrolytic
solution is held by the polymer compound, the electrolytic solution
may be directly used. In this case, the electrolytic solution
impregnates the separator 35.
The secondary battery including the gel electrolyte layer 36 is
manufactured, for example, by the following three procedures.
In the first manufacturing method, first, the cathode 33 and the
anode 34 are formed by procedures similar to those of the cathode
21 and the anode 22 in the first secondary battery. Specifically,
the cathode 33 is formed by forming the cathode active material
layer 33B on both faces of the cathode current collector 33A, and
the anode 34 is formed by forming the anode active material layer
34B on both faces of the anode current collector 34A. Subsequently,
a precursor solution containing an electrolytic solution, a polymer
compound, and a solvent is prepared. After the cathode 33 and the
anode 34 are coated with the precursor solution, the solvent is
volatilized to form the gel electrolyte layer 36. Subsequently, the
cathode lead 31 is attached to the cathode current collector 33A by
welding or the like, and the anode lead 32 is attached to the anode
current collector 34A by welding or the like. Subsequently, the
cathode 33 and the anode 34 provided with the electrolyte layer 36
are layered with the separator 35 in between and spirally wound.
After that, the protective tape 37 is adhered to the outermost
periphery thereof to form the spirally wound electrode body 30.
Finally, after the spirally wound electrode body 30 is sandwiched
between two pieces of the film package members 40, outer edges of
the package members 40 are bonded by thermal fusion bonding or the
like to enclose the spirally wound electrode body 30. At this time,
the adhesive films 41 are inserted between the cathode lead 31, the
anode lead 32 and the package member 40. Thereby, the secondary
battery illustrated in FIG. 4 to FIG. 6 is completed.
In the second manufacturing method, first, the cathode lead 31 is
attached to the cathode 33, and the anode lead 32 is attached to
the anode 34. Subsequently, the cathode 33 and the anode 34 are
layered with the separator 35 in between and spirally wound. After
that, the protective tape 37 is adhered to the outermost periphery
thereof, and thereby a spirally wound body as a precursor of the
spirally wound electrode body 30 is formed. Subsequently, after the
spirally wound body is sandwiched between two pieces of the film
package members 40, the outermost peripheries except for one side
are bonded by thermal fusion bonding or the like to obtain a
pouched state, and the spirally wound body is contained in the
pouch-like package member 40. Subsequently, a composition of matter
for electrolyte containing an electrolytic solution, a monomer as a
raw material for the polymer compound, a polymerization initiator,
and if necessary other material such as a polymerization inhibitor
is prepared, which is injected into the pouch-like package member
40. After that, the opening of the package member 40 is
hermetically sealed by thermal fusion bonding or the like. Finally,
the monomer is thermally polymerized to obtain a polymer compound.
Thereby, the gel electrolyte layer 36 is formed. Accordingly, the
secondary battery is completed.
In the third manufacturing method, the spirally wound body is
formed and contained in the pouch-like package member 40 in the
same manner as that of the foregoing second manufacturing method,
except that the separator 35 with both faces coated with a polymer
compound is used firstly. Examples of polymer compounds with which
the separator 35 is coated include a polymer containing vinylidene
fluoride as a component (a homopolymer, a copolymer, a
multicomponent copolymer or the like). Specific examples include
polyvinylidene fluoride, a binary copolymer containing vinylidene
fluoride and hexafluoropropylene as a component, and a ternary
copolymer containing vinylidene fluoride, hexafluoropropylene, and
chlorotrifluoroethylene as a component. As a polymer compound, in
addition to the foregoing polymer containing vinylidene fluoride as
a component, another one or more polymer compounds may be
contained. Subsequently, an electrolytic solution is prepared and
injected into the package member 40. After that, the opening of the
package member 40 is sealed by thermal fusion bonding or the like.
Finally, the resultant is heated while a weight is applied to the
package member 40, and the separator 35 is contacted with the
cathode 33 and the anode 34 with the polymer compound in between.
Thereby, the electrolytic solution impregnates the polymer
compound, and the polymer compound is gelated to form the
electrolyte layer 36. Accordingly, the secondary battery is
completed.
In the third manufacturing method, the swollenness of the secondary
battery is inhibited compared to the first manufacturing method.
Further, in the third manufacturing method, the monomer, the
solvent and the like as a raw material of the polymer compound are
hardly left in the electrolyte layer 36 compared to the second
manufacturing method. Thus, the formation of the polymer compound
is favorably controlled. Therefore, sufficient contact
characteristics are obtained between the cathode 33/the anode
34/the separator 35 and the electrolyte layer 36.
In the secondary battery, at the time of charge, for example,
lithium ions are extracted from the cathode 33, and are inserted in
the anode 34 through the electrolyte layer 36. Meanwhile, at the
time of discharge, for example, lithium ions are extracted from the
anode 34, and are inserted in the cathode 33 through the
electrolyte layer 36.
According to the second secondary battery, the anode 34 has the
structure similar to that of the anode 10 illustrated in FIG. 1.
Thus, the cycle characteristics are able to be improved while a
high capacity is obtained. Other effect of the second secondary
battery is similar to that of the foregoing anode 10.
2-3. Third Secondary Battery (Square Type)
FIG. 7 and FIG. 8 illustrate a cross sectional structure of a third
secondary battery. The cross section illustrated in FIG. 7 and the
cross section illustrated in FIG. 8 are orthogonal to each other as
the positional relation. That is, FIG. 8 is a cross sectional view
taken along line VIII-VIII illustrated in FIG. 7. The secondary
battery is a so-called square type battery and is a lithium ion
secondary battery in which a flat spirally wound electrode body 60
is contained in a package can 51 in the shape of an approximate
hollow rectangular solid.
The package can 51 is made of, for example, iron (Fe) plated by
nickel (Ni). The package can 51 also has a function as an anode
terminal. One end of the package can 51 is closed and the other end
of the package can 51 is opened. At the open end of the package can
51, an insulating plate 52 and a battery cover 53 are attached, and
thereby inside of the battery can 51 is hermetically closed. The
insulating plate 52 is made of, for example, polypropylene or the
like, and is arranged perpendicular to the spirally wound
circumferential face on the spirally wound electrode body 60. The
battery cover 53 is, for example, made of a material similar to
that of the battery can 51, and also has a function as an anode
terminal together with the package can 51. Outside of the battery
cover 53, a terminal plate 54 as a cathode terminal is arranged. In
the approximate center of the battery cover 53, a through-hole is
provided. A cathode pin 55 electrically connected to the terminal
plate 54 is inserted in the through-hole. The terminal plate 54 is
electrically insulated from the battery cover 53 with an insulating
case 56 in between. The cathode pin 55 is electrically insulated
from the battery cover 53 with a gasket 57 in between. The
insulating case 56 is made of, for example, polybutylene
terephthalate or the like. The gasket 57 is made of, for example,
an insulating material, and the surface thereof is coated with
asphalt.
In the vicinity of the rim of the battery cover 53, a cleavage
valve 58 and an electrolytic solution injection hole 59 are
provided. The cleavage valve 58 is electrically connected to the
battery cover 53. When the internal pressure of the battery becomes
a certain level or more by internal short circuit, external heating
or the like, the cleavage valve 58 is cleaved to suppress an
increase in internal pressure. The electrolytic solution injection
hole 59 is sealed by a sealing member 59A made of, for example, a
stainless steel ball.
In the spirally wound electrode body 60, a cathode 61 and an anode
62 are layered with a separator 63 in between, and are spirally
wound. The spirally wound electrode body 60 is shaped flat
according to the shape of the package can 51. The separator 63 is
located at the outermost circumference of the spirally wound
electrode body 60, and the cathode 61 is located just inside
thereof. FIG. 8 is a simplified view of the laminated structure of
the cathode 61 and the anode 62. The spirally winding number of the
spirally wound electrode body 60 is not limited to the number
illustrated in FIG. 7 and FIG. 8, but is able to be arbitrarily
set. A cathode lead 64 made of aluminum (Al) or the like is
connected to the cathode 61 of the spirally wound electrode body
60. An anode lead 65 made of nickel or the like is connected to the
anode 62. The cathode lead 64 is electrically connected to the
terminal plate 54 by being welded to the lower end of the cathode
pin 55. The anode lead 65 is welded and electrically connected to
the package can 51.
As illustrated in FIG. 7, in the cathode 61, a cathode active
material layer 61B is provided on a single face or both faces of a
cathode current collector 61A. In the anode 62, an anode active
material layer 62B is provided on a single face or both faces of an
anode current collector 62A. Structures of the cathode current
collector 61A, the cathode active material layer 61B, the anode
current collector 62A, the anode active material layer 62B, and the
separator 63 are respectively similar to the structures of the
cathode current collector 21A, the cathode active material layer
21B, the anode current collector 22A, the anode active material
layer 22B, and the separator 23 in the first secondary battery
described above. An electrolytic solution similar to that of the
separator 23 impregnates the separator 63.
The third secondary battery is able to be manufactured, for
example, as follows.
As in the foregoing first secondary battery, the cathode 61 and the
anode 62 are layered with the separator 63 in between and spirally
wound, and thereby the spirally wound electrode body 60 is formed.
After that, the spirally wound electrode body 60 is contained in
the package can 51. Next, the insulating plate 52 is arranged on
the spirally wound electrode body 60. The anode lead 65 is welded
to the battery can 51, the cathode lead 64 is welded to the lower
end of the cathode pin 55, and the battery cover 53 is fixed on the
open end of the battery can 51 by laser welding. Finally, the
electrolytic solution is injected into the package can 51 through
the electrolytic solution injection hole 59, and impregnates the
separator 63. After that, the electrolytic solution injection hole
59 is sealed by the sealing member 59A. The secondary battery
illustrated in FIG. 7 and FIG. 8 is thereby completed.
According to the third secondary battery, the anode 62 has the
structure similar to that of the anode 10 illustrated in FIG. 1
described above. Thus, the cycle characteristics are able to be
improved while a high capacity is obtained. Other effect of the
third secondary battery is similar to that of the foregoing anode
10.
3. Application of a Lithium Ion Secondary Battery
Next, a description will be given of an application example of the
foregoing lithium ion secondary battery.
Applications of the lithium ion secondary battery is not
particularly limited as long as the lithium ion secondary battery
is applied to a machine, a device, an instrument, an equipment, a
system (collective entity of a plurality of devices and the like)
or the like that is able to use the lithium ion secondary battery
as a drive power source, an electric power storage source for
electric power storage or the like. In the case where the lithium
ion secondary battery is used as a power source, the lithium ion
secondary battery may be used as a main power source (power source
used preferentially), or an auxiliary power source (power source
used instead of a main power source or used being switched from the
main power source). The main power source type is not limited to
the lithium ion secondary battery.
Examples of applications of the lithium ion secondary battery
include portable electronic devices such as a video camera, a
digital still camera, a mobile phone, a notebook personal computer,
a cordless phone, a headphone stereo, a portable radio, a portable
television, and a Personal Digital Assistant (PDA); a portable
lifestyle device such as an electric shaver; a storage equipment
such as a backup power source and a memory card; an electric power
tool such as an electric drill and an electric saw; a medical
electronic device such as a pacemaker and a hearing aid; a vehicle
such as an electrical vehicle (including a hybrid car); and an
electric power storage system such as a home battery system for
storing electric power for emergency or the like. It is needless to
say that application other than the foregoing applications may be
adopted.
Specially, the lithium ion secondary battery is effectively applied
to the electric power tool, the electrical vehicle, the electric
power storage system or the like. In these applications, since
superior battery characteristics (cycle characteristics, storage
characteristics, and load characteristics and the like) are
demanded, the characteristics are able to be effectively improved
by using the lithium ion secondary battery of the disclosure. The
electric power tool is a tool in which a moving part (for example,
a drill or the like) is moved by using the lithium ion secondary
battery as a driving power source. The electrical vehicle is a
vehicle that acts (runs) by using the lithium ion secondary battery
as a driving power source. As described above, a vehicle including
the drive source as well other than the lithium ion secondary
battery (hybrid car or the like) may be adopted. The electric power
storage system is a system using the lithium ion secondary battery
as an electric power storage source. For example, in a home
electric power storage system, electric power is stored in the
lithium ion secondary battery as an electric power storage source,
and the electric power is consumed according to needs. In the
result, various devices such as home electric products become
usable. E
EXAMPLES
Specific examples of the disclosure will be described in
detail.
Example 1-1
First, an equal amount of lithium hydroxide (LiOH) was reacted in
an aqueous solution containing 50% hydrofluorosilicic acid
(H.sub.2SiF.sub.6), and lithium fluorsilicate (Li.sub.2SiF.sub.6)
as neutralized salt was filtered and dried. Then, the obtained
lithium fluorosilicate was finely pulverized to obtain powder with
a particle size distribution of D.sub.50=10 .mu.m.
Next, 1000 g of the lithium fluorosilicate obtained above and 10 g
of vapor-grown carbon (VGCF) was dry-blended. The VGCF is fibrous
carbon with a length of several tens of .mu.m, and is added for
mainly reducing resistance in the active material layer. The
mixture was mixed with 200 g of N-methyl-2-pyrrolidone to which 20
g of polyvinylidene fluoride has been added, and stirred for 15 min
at a low speed (5 rpm). Next, 150 g of NMP in which 15 g of
hydroxypropyl methylcellulose as a thickening agent and a binder
has been dissolved was injected, and stirred for 10 min at a low
speed, and then further stirred for 20 min at a high speed (15
rpm). After that, 15 g of polyvinylidene fluoride powder was
injected and stirred for 15 min at a low speed to obtain paste
mixture slurry. Stirring was performed using a planetary mixer. A
current collector made of a copper foil with a thickness of 10
.mu.m was uniformly coated with the mixture slurry by a die-coating
intermittent coating method, and the mixture slurry was dried.
After that, the resultant was compression-molded at 120 deg C. to
obtain a volume density of 1.8 g/cm.sup.3 to form the active
material layer, thereby obtaining an electrode. Drying was
performed by applying hot air of a temperature of 80 deg C. to 110
deg C., both inclusive, using a spray dryer. Further, the electrode
obtained as described above was heat-treated at a temperature of
160 deg C. to 190 deg C., both inclusive, in a non-oxidizing
nitrogen gas or inert gas atmosphere, and carbonization of the
thickening agent, removal of crystal water, and active material
surface coating by thermal decomposition of polyvinylidene fluoride
were performed. As a result of heat treatment being performed at
160 deg C. to 190 deg C., both inclusive, removal of crystal water,
carbonization of the thickening agent, and surface coating of the
active material by polyvinylidene fluoride were favorably
performed, without causing deformation of the electrode shape due
to hardening of polyvinylidene fluoride.
Next, using the electrode, a coin-type test cell (diameter of 20 mm
and thickness of 1.6 mm) having a structure illustrated in FIG. 9
was prepared. The test cell uses the foregoing electrode punched to
form a pellet with a diameter of 16 mm as a test electrode 71. The
test electrode 71 was contained in a package can 72, and a counter
electrode 73 was bonded to a package cup 74. Then, the test
electrode 71 and the counter electrode 73 were layered with a
separator 75 impregnated with an electrolytic solution
therebetween, and caulked with a gasket 76. That is, in the test
electrode 71, an active material layer 71B made by the surface of
an active material composed of lithium fluorosilicate and
vapor-grown carbon being coated by polyvinylidene fluoride is
provided on a current collector 71A composed of a copper foil. The
active material layer 71B is arranged to oppose the counter
electrode 73 with the separator 75 therebetween. Here, a lithium
metal was used as the counter electrode 73 and a porous film made
of polyethylene was used as the separator 75. As the electrolytic
solution, an electrolytic solution obtained by dissolving
LiPF.sub.6 as an electrolyte salt into a mixed solvent of ethylene
carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC),
and vinyl ethylene carbonate (VEC) mixed at a mass ratio of
50:30:17:3. Here, the content of LiPF.sub.6 to the mixed solvent
was 1.0 mol/kg.
Example 1-2
A test cell (FIG. 9) of Example 1-2 was fabricated in the same
manner as that of Example 1-1, except that silicon difluoride
(SiF.sub.2) was used instead of lithium fluorosilicate as the
active material.
Example 1-3
A test cell (FIG. 9) of Example 1-3 was fabricated in the same
manner as that of Example 1-1, except that graphite was used
instead of lithium fluorosilicate as the active material. The
graphite used here is graphite particles obtained by pulverizing
natural graphite using a hammer mill, a pin mill, a ball mill, a
jet mill, or the like. For example, in the case where a hammer mill
is used, the natural graphite is preferably pulverized for 20 min
or more at a rotation speed of 4000 rpm to 5000 rpm, both
inclusive. Supply of natural graphite and discharge of the
pulverized graphite particles are preferably performed by a method
in which the natural graphite and the graphite particles are caught
in an air current.
For the test cells of Examples 1-1 to 1-3 fabricated as above, the
discharge capacity (mAh/g) was examined. Specifically, the
discharge capacity was determined as follows. First, the test cell
was constant-current charged with a constant current 1 C until the
equilibrium potential reached 0 V to lithium, and was further
constant-voltage charged with a constant voltage 0 V until the
total amount of time from the start of constant-current charge
reached four hours. After that, the test cell was discharged with a
constant current of 1 C until the equilibrium potential reached 1.5
V to lithium, and the discharge capacity (mAh/g) per unit mass, in
which the mass of the copper foil current collector and the binder
was subtracted from the mass of the test electrode 71, was
measured. Here, 1C is a current value at which theoretical capacity
is completely discharged in one hour. The discharge capacity
calculated as described above is based on the equilibrium
potential. Thereby, the discharge capacity reflects the unique
characteristics of the materials composing the active material
layer of the test electrode 71. In addition, charge herein refers
to insertion reaction of lithium into the active material layer
71B. The results of the obtained discharge capacity (mAh/g) are
illustrated in Table 1. Further, FIG. 10 illustrates a discharge
curve in the test cells of Examples 1-1 to 1-3. In FIG. 10, the
discharge capacity (mAh/g) per unit mass is the horizontal axis,
and the equilibrium potential (V) to lithium is the vertical
axis.
TABLE-US-00001 TABLE 1 Coin-type test cell Active material of test
Discharge capacity per Table 1 electrode unit weight (mAh/g)
Example 1-1 Lithium fluorosilicate 1275 (Li.sub.2SiF.sub.6) Example
1-2 Difluoride (SiF.sub.2) 680 Example 1-3 Graphite 320
As illustrated in FIG. 10 and Table 1, it was confirmed that, in
the case where lithium fluorosilicate (Li.sub.2SiF.sub.6) was used
as the active material (Example 1-1), about twice the discharge
capacity of the case where difluoride (SiF.sub.2) was used (Example
1-2) and about four-times the discharge capacity of the case where
graphite was used (Example 1-3) were obtained.
Example 2-1
Next, a cylindrical secondary battery including the anode 22 and
the cathode 21 illustrated in FIG. 2 was fabricated. The anode 22
was an anode in which both faces of the anode current collector 22A
were coated with mixed slurry similar to that used for the
electrode in the foregoing Example 1-1, and the mixed slurry was
dried. The resultant was then compression-molded by a rolling press
machine or the like to form the anode active material layer 22B,
and the anode lead 26 was then attached to one end of the anode
current collector 22A. A strip-shaped copper foil with a thickness
of 10 .mu.m was used as the anode current collector 22A. The volume
density of the anode active material layer 22B was 1.80 g/cm.sup.3,
and the thickness at one face of the anode active material layer
22B was 80 .mu.m.
The cathode 21 was fabricated as follows. Specifically, first,
lithium carbonate (Li.sub.2CO.sub.3) and cobalt carbonate
(CoCO.sub.3) were mixed at a molar ratio of
Li.sub.2CO.sub.3:CoCO.sub.3=0.5:1. After that, the mixture was
fired in the air at 900 deg C. for 5 hours. Thereby, lithium-cobalt
composite oxide (LiCoO.sub.2) was obtained. X-ray diffraction was
performed on the obtained LiCoO.sub.2, and the peak thereof matched
well the peak of LiCoO.sub.2 registered in the JCPDS (Joint
Committee of Powder Diffraction Standard) file. Next, the
lithium-cobalt composite oxide was pulverized into a powder state
with a particle size of 15 .mu.m at 50% cumulative volume obtained
by laser diffraction method, thereby obtaining the cathode active
material.
Then, 95% by mass of lithium-cobalt composite oxide powder and 5%
by mass of lithium carbonate (Li.sub.2CO.sub.3) powder were mixed,
and 91% by mass of the mixture, 6% by mass of graphite as a
conductor, and 3% by mass of polyvinylidene fluoride as a binder
were mixed. The resultant was dispersed in N-methyl-2-pyrrolidone
to obtain cathode mixture slurry. Next, both faces of the cathode
current collector 21A made of a strip-shaped aluminum foil with a
thickness of 15 .mu.m were uniformly coated with the cathode
mixture slurry, which was dried. After that, the resultant was
compression-molded by a roll pressing machine or the like to form
the cathode active material layer 21B, thereby fabricating the
cathode 21. At this time, the thickness at one face of the cathode
active material layer 21B was 80 .mu.m, and volume density was 3.55
g/cm.sup.3. Then, the cathode lead 25 made of aluminum was attached
to one end of the cathode current collector 21A. At this time, the
secondary battery is a lithium ion secondary battery in which the
capacity of the anode 22 is expressed based on insertion and
extraction of lithium. In other words, the thickness of the cathode
active material layer 21B and the thickness of the anode active
material layer 22B were each adjusted such that lithium is
prevented from being precipitated on the anode 22 in the fully
charged state.
After the cathode 21 and the anode 22 were respectively fabricated,
with the separator 23 made of a micro-porous polyethylene stretch
film with a thickness of 20 .mu.m therebetween, the anode 22, the
separator 23, the cathode 21, and the separator 23 were layered in
this order, and wound numerous times to fabricate the
jellyroll-shaped spirally wound electrode body 20. Next the
spirally wound electrode body 20 was sandwiched between the pair of
insulating plates 12 and 13. The anode lead 26 was welded to the
battery can 11 and the cathode lead 25 was welded to the safety
valve mechanism 15. The spirally wound electrode body 20 was
contained within the battery can 11. Subsequently, the electrolytic
solution was injected into the battery can 11 by a decompression
method or the like, and impregnated the separator 23. Finally, the
battery cover 14 was caulked to the battery can 11 with the gasket
17, thereby the cylindrical secondary battery having an outer
diameter of 18 mm and a height of 65 mm was fabricated.
At this time, as the electrolytic solution, an electrolytic
solution obtained by dissolving LiPF.sub.6 as an electrolyte salt
into a mixed solvent of ethylene carbonate (EC), diethyl carbonate
(DEC), propylene carbonate (PC), and vinyl ethylene carbonate (VEC)
mixed at a mass ratio of 50:30:17:3 was used. Here, the content of
LiPF.sub.6 to the mixed solvent was 1.0 mol/kg.
Example 2-2
A cylindrical secondary battery (FIG. 2) of Example 2-2 was
fabricated in the same manner as that of Example 2-1, except that
silicon difluoride (SiF.sub.2) was used instead of lithium
fluorosilicate as the anode active material.
Example 2-3
A cylindrical secondary battery (FIG. 2) of Example 2-3 was
fabricated in the same manner as that of Example 2-1, except that
graphite was used instead of lithium fluorosilicate as the anode
active material.
Example 2-4
A cylindrical secondary battery (FIG. 2) of Example 2-4 was
fabricated in the same manner as that of Example 2-1, except that
lithium fluorosilicate and graphite in equal amounts (500 g) were
used as the anode active material.
Example 2-5
A cylindrical secondary battery (FIG. 2) of Example 2-5 was
fabricated in the same manner as that of Example 2-1, except that
polyvinylidene fluoride was not added as a fluorine-containing
compound to the anode active material layer 22B, and heat treatment
was not performed.
For the secondary batteries of Examples 2-1 to 2-5 fabricated as
above, charge and discharge operation was performed a plurality of
times, and the battery capacity and the cycle characteristics
(discharge capacity retention ratio) were examined. At this time,
regarding charge, after constant-current charge was performed at a
constant current of 0.7 C until the battery voltage reached 4.2 V,
constant-voltage charge was then performed at a constant voltage of
4.2 V until the current density reached 0.03 mA/cm.sup.2. Regarding
discharge, constant-current discharge was performed at a constant
current of 1 C until the battery voltage reached 3.0 V. With the
battery capacity being the initial discharge capacity (discharge
capacity at the first cycle), the cycle characteristics (discharge
capacity retention ratio) is a ratio of the discharge capacity of
the 500th cycle to the initial discharge capacity (discharge
capacity at the first cycle), that is (discharge capacity at the
500th cycle/discharge capacity at the first cycle).times.100(%).
The measurement results of the battery capacity (mAh) and the cycle
characteristics (discharge capacity retention ratio (%)) are
illustrated in Table 2. FIG. 11 illustrates the changes in
discharge capacity in the secondary batteries of the Examples 2-1
to 2-5. In FIG. 11, the number of cycles (cycle) is the horizontal
axis, and the battery capacity (mAh) is the vertical axis. In
addition, the expansion rate after 500 cycles with the state before
charge and discharge as reference was measured, and is also
illustrated in Table 2. The expansion rate herein refers to the
thickness of the anode after 500 cycles to the thickness of the
anode before charge and discharge.
TABLE-US-00002 TABLE 2 cylindrical-type Composition ratio of
Discharge anode active material Battery capacity (% by weight)
capacity retention Expansion Table 2 Li.sub.2SiF.sub.6 SiF.sub.2
Graphite (mAh) ratio (%) ratio (%) Example 100 0 0 950 84.2 30 2-1
Example 0 100 0 800 61.5 225 2-2 Example 0 0 100 600 75.0 29 2-3
Example 50 0 50 845 82.8 29.5 2-4 Example 100 0 0 950 10.2 105
2-5
As illustrated in FIG. 11 and Table 2, in the case where lithium
fluorosilicate is included as the anode active material (Examples
2-1, 2-4, and 2-5), the cycle characteristics are able to be
improved compared to the case where only graphite is used as the
anode active material. In particular, in the case where the anode
active material layer 22 contains polyvinylidene fluoride (Examples
2-1 and 2-4), it is confirmed that the cycle characteristics are
able to be further improved while retaining almost the same low
expansion ratio. This is considered to be a result of reaction
between lithium fluorosilicate and acid impurities being inhibited
since the lithium atoms of lithium fluorosilicate
(Li.sub.2SiF.sub.6) forming an octahedron structure and the
fluorine atoms of polyvinylidene fluoride form an electrostatically
stable state and provide sterical hinder. Meanwhile, in Example
2-2, although the anode active material layer 22 contains silicon
difluoride and polyvinylidene fluoride, deterioration of the cycle
characteristics was not able to be prevented. A reason for this is
considered that the anode active material because SiF.sup.- and
dissolved, and expansion due to sudden excessive formation of the
SEI coating occurred, as a result of increased concentration of
acidic impurities (such as HF) within the electrolytic solution
accompanying increase in the number of cycles.
In addition, for the secondary batteries of Examples 2-1 to 2-5,
the electric potentials of the cathode 21 and the anode 22 with the
oxidation-reduction potential of lithium as reference were
measured, and are illustrated in Table 3. Further, the electric
potentials of the cathode 21 and the anode 22 (with the
oxidation-reduction potential of lithium as reference) in the case
where the maximum battery voltage at the time of charge is 4.35 V
were similarly measured, and are also illustrated in Table 3.
TABLE-US-00003 TABLE 3 Cylindrical-type Charge voltage Charge
voltage Composition ratio of 4.2 V 4.35 V anode active material
Cathode Anode Cathode Anode (% by weight) potential potential
potential potential Table 3 Li.sub.2SiF.sub.6 SiF.sub.2 Graphite
(V) (V) (V) (V) Example 100 0 0 4.25 0.05 4.39 0.04 2-1 Example 0
100 0 4.35 0.15 4.47 0.12 2-2 Example 0 0 100 4.3 0.1 4.43 0.08 2-3
Example 50 0 50 4.28 0.08 4.41 0.06 2-4 Example 100 0 0 4.25 0.05
4.39 0.04 2-5
As illustrated in Table 3, in the case where lithium fluorine
silicate is included as the anode active material (Examples 2-1,
2-4, and 2-5, the cathode potential and the anode potential are
both positioned toward the base side by 0.4 V to 0.5 V, both
inclusive, compared to the case where only graphite is used as the
anode active material. Thus, lithium precipitation in the anode is
inhibited, and the secondary battery becomes further structurally
stable. In addition, since the cathode potential is less than 4.4 V
with the oxidation-reduction potential of lithium as reference,
oxidative decomposition of the electrolytic solution solvent does
not easily occur. Oxidative decomposition of a solvent such as this
may caused deterioration of various characteristics as secondary
reaction factor inhibiting charge and discharge reaction. However,
such concern does not arise in the present disclosure.
From the results of the foregoing respective examples, it was found
that according to the lithium ion secondary battery of the
disclosure, higher capacity and improved cycle characteristics are
able to be actualized by use of lithium fluorosilicate as the anode
active material. In particular, if a coating is formed on the anode
active material in advance by a fluorine-containing compound such
as polyvinylidene fluoride, it is confirmed that further superior
cycle characteristics are able to be obtained.
The disclosure has been described with reference to the embodiments
and the examples. However, the disclosure is not limited to the
foregoing embodiments and the foregoing examples, and various
modifications may be made. For example, in the foregoing
embodiments and examples, the description has been given of the
case in which the battery structure is the cylindrical type, the
laminated film type, or the square type, and of the case in which
the battery element has the spirally wound structure. However, the
battery structure is not limited thereto, and the disclosure is
similarly applicable to a case that the battery structure is a
coin-type or a button-type, or a case that the battery element has
a laminated structure or the like.
It should be understood that various changes and modifications to
the presently preferred embodiments described herein will be
apparent to those skilled in the art. Such changes and
modifications can be made without departing from the spirit and
scope and without diminishing its intended advantages. It is
therefore intended that such changes and modifications be covered
by the appended claims.
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